1. Field of Invention
The present invention relates to an innovative human-portable Self-calibrated Azimuth and Attitude Accuracy Enhancing Method and System (SAAAEMS) for a human-portable, tripod mounted, or handheld pointing system. The technical approach of the SAAAEMS implementation is based on an innovative gyrocompassing method of an inertial navigation system (INS). This SAAAEMS approach is based on fully auto-calibration self-contained INS principles, not depending on magnetometers for azimuth/heading determination, and thus the system outputs and performance are not affected by the environmental magnetic fields. This SAAAEMS is not a conventional strapdown INS. In order to reduce the system size and cost, this new innovative methods and algorithms are used for SAAAEMS system configuration and integration. Compared to a conventional INS for gyrocompassing, American GNC Corporation's (AGNC) approach uses a smaller number of high accuracy sensors: SAAAEMS uses only one 2-axis high accuracy gyro (for example, one DTG) instead of 3-axis; the third axis gyro is a MEMS gyro. It uses only 2 high accuracy accelerometers instead of 3, since the two accelerometers are used only for gyrocompassing not for navigation. These two changes to the conventional INS system configuration remarkably reduce the whole system size and cost. These 2-DOF inertial angular rate measurements and 2-DOF acceleration (gravity) measurements are adequate for the SAAAEMS gyrocompassing process that will determine the azimuth with respect to true north and the module's vertical attitude (elevation/pitch and roll). The third MEMS gyro enables the SAAAEMS to work in the dynamic state.
2. Description of Related Arts
The SAAAEMS is applicable to a broad range of pointing, control and navigation systems. Many traditional approaches and systems can be used for pointing, but they have disadvantages. An extensive survey of the existing methodologies corresponding to pointing, target acquisition, attitude determination, geolocation and processing has been performed by the inventor. A brief description of the related and existing technology survey is presented next.
High-Accuracy triad or 2 accelerometers for vertical angle measurement. This is a conventional approach based on the measurement of the earth's gravity to determine the sensor frame's attitude with respect to the gravitational force direction. At present, the high-end MEMS accelerometers can achieve an accuracy of 1 to 2 mils on a stationary base.
High-Accuracy triad magnetometers data for azimuth measurement usually need to be combined with the accelerometer data to obtain accurate azimuth at different elevation/tilt angles. The advantage of the magnetometers is that the sensor module size can be very small and it is also of low cost. The downside of the magnetometers is that they can be easily interfered with by magnetic or ferrous objects, such as buildings, vehicles, power lines, buried pipes and even a soldier's individual combat load. At present, the best digital magnetometers can achieve a heading accuracy of 0.5 degrees, under ideal application environment and with good calibration and compensation, including magnetic declination angle compensation. Higher accuracy and reliability for the magnetometer heading measurement is very difficult to obtain currently.
Use IMU (inertial measurement unit, consists of 3 gyros and 3 accelerometers) to measure azimuth, pitch/elevation, and roll angles through the conventional gyrocompassing approach. The first advantage of this approach is that it is autonomous or self-calibrated. It is not affected by magnetic fields and needs no external information, such as initial conditions or external reference. Traditionally, the required high accuracy gyros and accelerometers are expensive and big in size. The technology advancements in the past 10 years have transformed the inertial sensors, especially the advances of the MEMS inertial sensor. Also, advanced processing algorithms and real-time calibration methods realized by powerful computers/microprocessors can lower the accuracy requirements for the sensors. However, at present, the MEMS gyros are not accurate enough for the gyrocompassing approach and the conventional implementation based on high accuracy (usually inertial grade or better) gyros can be too expensive and oversized for a handheld system.
Use of a GPS attitude determination approach to measure azimuth and pitch/elevation angles. This method leads to an inexpensive system. However, its size can not be small with current technologies. It needs at least 2 separated GPS antennas and their separation may need to be 2 to 3 meters. Also, it is not self-contained. If the GPS signals are blocked or jammed, the system fails.
Reference point method. This method needs a DGPS receiver and the laser ranger finder needs to be mounted on an angular header to perform accurate relative angle measurements (goniometer). To determine the azimuth of any target, the user must establish a reference baseline first. An object is chosen as the survey control point with coordinates determined by the DGPS to an accuracy of 10 to 20 cm. Then the Accurate Heading (AH) system is positioned at a separate point and the true azimuth of the line between the two points is obtained by their DGPS-provided geographical positions. Using this baseline as a reference the true north azimuth of the target is determined. The disadvantage of this method is that it is not convenient and not suitable for mobile applications.
If used on a dynamic moving platform, the Coremicro® AHRS/INS/GPS unit (AINSGPS) can also provide accurate heading and attitude angle measurements. Unfortunately, with the use of the AH system most of the time on stationary or slow moving platforms (such as with a handheld binocular laser ranger finder), this approach is not suitable for the AH system application.
The advantage of the magnetometers is that the sensor module size can be very small and it is also of low cost. The downside of the magnetometers is that they can be easily interfered with by magnetic or ferrous objects, such as buildings, vehicles, power lines, buried pipes and even a soldier's individual combat load. At present, the best digital magnetometers can achieve a heading accuracy of 0.5 degrees, under ideal environment and with good calibration and compensation, including magnetic declination angle compensation. Higher accuracy and reliability for the magnetometer heading measurement is very difficult to obtain currently. The GPS attitude determination approach leads to an inexpensive system. However, its size can not be small with current technologies. It needs at least 2 separated GPS antennas and their separation may need to be 2 to 3 meters. Also, it is not self-contained. If the GPS signals are blocked or jammed, the system fails.
Based on the existing technology surveys and careful trade-off studies, we have established a good justification for the invention of AGNC's non-magnetometer gyrocompassing-based Self-calibrated Azimuth and Attitude Accuracy Enhancing Method and System (SAAAEMS) technology. It uses only one 2-axis tactical-grade gyro instead of a three-axis navigation grade IMU, plus one MEMS gyro, and two MEMS accelerometers, for attitude and azimuth determination.
The SAAAEMS is based on gyrocompassing approaches. Gyrocompassing is a fundamental method for INS initial alignment. It is a self-contained alignment mainly for ground-based applications. The key advantage of this approach is that it is autonomous or self-calibrated. It is not affected by magnetic fields and needs no external information, such as initial conditions or external reference. Traditionally, the high accuracy gyros and accelerometers are expensive and big in size. But the technology advancements in the past 10 years have transformed the inertial sensors, especially the advances of the new technology based inertial sensors, such as Hemispherical Resonator Gyros (HRG), MEMS quartz (vibration) gyros and MEMS quartz accelerometers. The small 2-axis tactical or inertial grade dynamically tuned gyros (DTG) are also suitable for the AH systems. Also, advanced processing algorithms and real-time calibration methods realized by powerful computers/microprocessors can lower the accuracy requirement for the sensors.
For SAAAEMS implementation, we use a 2-axis DTG gyro in the SAAAEMS system because it is much smaller than a FOG gyro with the same accuracy. Compared to RLG and FOG, the DTG may not be best in accuracy and robustness, but for most applications, such as the SAAAEMS system, it still has more advantages:                Small size        Low cost        Adequate accuracy, if properly modeled and compensated/calibrated.        
Our SAAAEMS system also has a third MEMS gyro to enhance system dynamic (angular motion) performance. This SAAAEMS system features small size, low cost, low power consumption, and easy automatic calibration, with satisfactory accuracy.